Overbore

Overbore cartridges are those with a relatively large case volume or case capacity, coupled with a relatively small diameter bullet.

The case volume or case capacity and barrel bore area can be mathematically related to obtain a case volume to bore area ratio in metric or imperial units.

where:

  • = the cartridge case internal volume or case capacity (in ml or (for non-metric users) grains of water)
  • = barrel bore cross section area (in cm2 or in2)

The higher the Oratio result, the more overbore a cartridge will be. As the ratio is expressed in units of length, relatively high Oratio is a good predictor of suitability for relatively long barreled guns.

Oratio is also used to predict barrel life in cartridges of the same caliber, but not of different calibres, since the ratio is an extensive quantity that does not correlate to temperature or pressure (e.g. a .50 cal straight cartridge may have the same overbore as a highly necked down .17 cal cartridge).

Comparative index for various rifle cartridges

ChamberingCase capacity (ml / cm3)Abore (cm2)Oratio (cm)
7.92×33mm Kurz2.220.51784.29
7.62×39mm2.310.47994.81
7.62×45mm2.790.47495.87
5.45×39mm1.750.23997.29
.223 Remington1.870.25037.47
.303 British3.640.48077.57
9.3×62mm5.060.66327.63
7.65×53mm Mauser3.700.48367.65
.308 Winchester3.640.47517.66
7.92×57mm Mauser4.090.51787.90
7.5×54mm French3.760.47177.97
5.8×42mm2.110.26427.99
8mm Lebel4.280.52408.16
6mm Norma BR2.450.29528.30
9.3×64mm Brenneke5.710.66328.61
7.62×54mmR4.160.47998.67
.375 Holland & Holland Magnum6.180.70168.81
6.5×47mm Lapua3.110.34598.99
7.5×55mm Swiss GP 114.220.46339.11
.30-06 Springfield4.420.47559.30
7×57mm3.900.40419.65
6.5mm Creedmoor3.400.34669.81
.300 Winchester Short Magnum4.990.475210.50
8×68mm S5.580.517810.78
6.5×55mm3.750.343610.91
7×64mm4.480.402911.12
.243 Winchester3.510.292512.00
.338 Norma Magnum6.950.568912.22
.300 Winchester Magnum5.930.473212.53
.338 Lapua Magnum7.400.568613.01
7mm Remington Magnum5.310.403913.15
.300 Norma Magnum6.750.475114.21
.50 BMG18.971.296314.63
.300 Remington Ultra Magnum7.140.474015.06
.300 Lapua Magnum7.340.475115.45
.257 Weatherby Magnum5.450.330716.48
.30-378 Weatherby Magnum8.640.474818.20
6.5-300 Weatherby Magnum6.360.345218.42
14.5×114mm42.531.651325.76

The bore cross section areas "Q" used in the calculations were taken from the appropriate C.I.P. data sheets.

The intermediate cartridges 7.92×33mm Kurz, 7.62×39mm, 7.62×45mm, 5.45×39mm, .223 Remington/5.56×45mm NATO and 5.8×42mm stand out as having relatively low sub 8 Oratio's.

References

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